A variable optical attenuator (VOA) is an electro-optical component capable of attenuating an optical power by a varied amount of attenuation based on a user's requirements by applying an electrical control signal. Variable optical attenuators are widely used to control or regulate optical power levels in optical telecommunication systems. For example, an optical attenuator is needed in optical telecommunication network laser sources to regulate an output optical power. In an other example, an optical attenuator is needed in laser detector when an optical signal with an excessive power level, that is, greater than a predetermined power level, is received. In a further example, an optical attenuator is needed to balance the optical power level among multiple channels in the EDFA (Erbium-Doped Fiber Amplifier) module.
Variable optical attenuators have been produced by various methods, including electro-optical polarization rotation, waveguide index change, bulk mechanical and micro mechanical light beam blocking or steering. Among MEMS VOAs, electro-static based devices are a most common approach. However, their need for a high electrical field to generate sufficient actuation force results in the requirement of costly hermetic packaging. Electro-thermal actuation is also used in VOAs. Nevertheless, these devices are large in size, the response time for thermal structures is relatively slow, and the device's thermal, control is a significant challenge for the packaging design. Previous designs that use electro-magnetic force have eliminated the need for the expensive hermetic packaging, but they need external magnets. Also, VOAs made by these designs are subject to drift of both time and background temperature dependence which prevents them to be used in general applications.
Because the attenuation of the optical signal is highly sensitive to the position of the beam blocking or steering mechanism, any change of the device characteristics with time and background temperature, for example, material Young's modulus, resistance, thermal expansion mismatch between different layers, will introduce drift to the attenuation signal. Further the conventional MEMS variable optical attenuators have a substantial disadvantage of signal drift and undesirably large size.
An improved MEMS VOA design is disclosed in U.S. Pat. No. 8,666,218, which is incorporated by reference herein in its entirety and for all purposes. However, for a compact thermal actuated VOA, for example, one using highly doped silicon, during the thermal equilibrium procedure of the device itself or environmental temperature change, the drift could be more than 1 dB. The increased resistance leads to a decreased input power to the actuator when the input voltage holds constant. This leads to lack of usefulness of the device, and increase cost of the fabrication.
Therefore, the market needs system and methods for thermal compensation of an improved MEMS VOA design.